With the rapid development of the science and technology, when active circuits operate in high-speed and operating frequencies thereof are increased, the active circuits require prompt supply of transient currents from the power distribution network (PDN) of package or printed circuit boards (PCBs). Then, simultaneous switching noises are generated to influence the normal operation of chips, and it is a problem which should be urgently solved. The conventional technology completely isolates the PCBs or a packaged structure by etching the slots between the power or ground layers, and it has been proved that it is very effectual to restrain the noise whose frequency is lower than GHz. Furthermore, it may design suitable slot structures to suppress the noise propagation within some specific frequency bands.
When a signal line passes through the slot structures, the discontinuous reference surface will result in serious degradation of signal quality because of the slot structures. Furthermore, if the frequency of passing signal thereof meets the resonance frequency of the slot structures, a part of the transmitted EM energy will excite the antenna-like slot to radiate outwards. To solve this kind of problems, the conventional technology employs the differential signal lines to transmit the signals. However, in the general PCB or package structure, it cannot avoid the asymmetry between two signal lines. For example, the differential signal lines may be bent due to the requirements of routing and this may cause unwanted common-mode noises propagating along the differential lines. The return current of the common-mode noise mainly passes through a reference surface below the differential signal transmission lines. Like the case of single-ended signal line, the energy of common-mode noise may radiate considerable energy by the slot structures, to generate the problems of the EM interference, etc.
The conventional technology is connecting two sides of the reference surface below and adjacent to the signal transmission lines by metal sheets, such that a direct-current (DC) level connection is formed between metal surfaces at two sides of the slot to provide a good electric connection, for solving the problem of the EM radiation generated by the signal transmission lines crossing over the gap or the slot of the reference surface. However, the conventional technology will break the original mechanism of restraining the noise transmission built by the slots. In addition, the conventional technology employs the DC connection, thus it is not adapted to a condition of two power layers with different voltage levels.
Currently, a common resolution is employing a SMT (surface mounted technique) capacitor, which are two terminals of connected to two ends of a slot and are adjacent to the signal transmission lines. Since the capacitor is electrical short in high frequencies, it can provide an excellent return current path when operating in the high frequencies. However, the parasitic effect of the SMT capacitor, such as the equivalent series inductors or resistors in series, will greatly influence the performances of the SMT capacitor, and the operation frequency thereof cannot be above GHz. In addition, the needs of SMT capacitor also increase the cost of the implementation.